Absorbency is a primary end-use property of tissue and towel products. Tissue and towel producers may capitalize upon increases in absorbency to market new and improved products or to reduce the amount of cellulose fiber used to achieve a certain level of absorbency. Super-absorbent materials have been developed that increase the absorbency of absorbent articles, such as tissue and towel products. For example, super-absorbent polymers (sometimes collectively referred to herein as “SAP”) are used in many industries and applications, such as medical, food, and agricultural industries. These materials also have utility in many consumer products including disposable absorbent articles, such as diapers, incontinent pads, feminine care products, tissues, and paper towels. SAPs are typically capable of absorbing 30 to over 200 times their weight in fluid. For certain applications, the trend is to provide thinner, more compact absorbent articles, which is generally contingent on the ability to develop relatively thin absorbent cores that can absorb, distribute, and store large quantities of fluid.
Although certain other polymer types provide super-absorbent properties, the predominant commercial products are partially neutralized, cross-linked polyacrylic acids, partly because of their cost efficiency. Some factors affecting super-absorbent capacity of such polymers include hydrophilicity, degree of cross-linking, and presence of dissociated ions. Hydrophilic polymers are used because they hydrate well and effectively form hydrogen bonds with water. Cross-linking is important because it prevents infinite swelling and eventual polymer dissolution. Too much cross-linking generally restricts swelling and thus decreases performance. Presence of ions provide charge repulsion to help the polymer matrix expand and also drive osmotic pressure effects to force more water into the polymer matrix.
Examples of SAPs include polyacrylates and their sodium, potassium, and lithium salts and polyacrylamide with a potassium salt base (e.g., EP 0992250 A2 and U.S. Pat. No. 6,984,419 B2). SAPs are typically designed to resist humidity, but will swell when put in intimate contact with water. They are usually prepared by either one of two methods. The first method involves sufficiently cross-linking emulsion or aqueous solution polymers to make them water insoluble, while retaining their ability to swell in water. The second method is modifying water-insoluble polymers with pendant hydrophilic groups to induce swelling when in contact with water.
SAPs are available in a particulate or powder form. In the case of diaper construction, SAPs are sifted into the absorbent core. The absorbent core is sandwiched between a fluid pervious topsheet and a fluid impervious backsheet. The incorporation of particulate SAP tends to generate dust from the SAP fines. Further, conventional absorbent articles have the limitation of the SAP not being sufficiently immobilized and are thus free to migrate and shift during the manufacturing process, shipping/handling, and/or use. Movement of the SAP particles during manufacture can lead to handling losses as well as improper distribution of the particles.
Powdered form SAPs need to be applied to a dry substrate thus necessitating a converting operation. SAPs do not bind well to a dry sheet, which creates a new set of problems when attempting to localize or evenly disperse the SAP. Absorbency problems also occur when the SAP particles migrate prior to, during, and after swelling. This inability to fix the particles at optimum locations leads to insufficient fluid storage in one area and over-capacity in other areas.
The fluid transport properties of the gel layer formed as a result of the swelling SAP particles in the presence of fluids is extremely important. Although the formation of a SAP gel layer fluid barrier, known as “gel blocking” is desirable for some applications, such as for use in cables, the formation of gel layers in disposable absorbent products is undesirable since it greatly reduces the efficiency of the SAP and causes “sliminess” or “clumpiness” when wetted. Thus, the advantages of being able to fixate SAP particles in place are apparent and several ways of accomplishing such have been suggested.
There are many cross-linkable water soluble/swellable polyacrylate-based compositions. However, consistent with the fact that commercially available SAP tends to be in granular, particulate, or powdered form, the impetus of the prior art is aimed at making highly viscous emulsions and dispersions that are subsequently dried, masticated, pulverized, or ground to the desired size. For example, U.S. Pat. No. 4,914,170 relates to super-absorbent polymeric compositions prepared from a monomer including acrylic acid and a second hydrophilic monomer, which can be a soluble salt of beta-acryloxypropionic acid. The pH of the aqueous monomer solution is typically adjusted to substantially exclude free acid, and the aqueous monomer solution is coated onto a heated surface to both polymerize the monomer and dry the resulting hydrogel.
As industry recognized the deficiencies of particulate SAPs, aqueous based super-absorbent polymer compositions began to be developed. With the advent of better super-absorbent polymers, a need exists to develop methods of using these polymers. In particular, a need exists to develop methods of incorporating these polymers into tissue, paper towel, and other absorbent substrates to increase their absorptive capacity.